The specification of regions of magnetic field homogeniety has been a limiting aspect for the achievable precision of high resolution magnetic resonance phenomena. For analytic nuclear magnetic resonance spectrometers departures from homogeniety are typically less than one part in 3.times.10.sup.-9 over a volume of the order of 1 cm. For the purposes of NMR imaging apparatus one wishes to produce magnetic field gradients which are precise and reproducible over volumes substantially similar to that of the human body. Both of these applications require control of magnetic field gradients.
Control of aberrant gradients (shimming) is ordinarily accomplished by addition of an equal magnitude gradient of opposite sign or direction to cancel the aberration. Gradients in several different directions necessitate corresponding corrections and high quality magnet systems are characterized by a number of shimming coils to correct undesired spatial dependences of the magnetic field and in modern NMR imaging systems, to establish desired spatial dependence. External to the magnet structure the dependences which commonly appear may be considered as concommitants of the multipole expansion of the magnetic field. Therefore, when a zero-gradient spatial dependence of the magnetic field is desired over a microscopic volume element, the spherical harmonic functions are to be synthesized by a system of shimming coils acting to cancel the high order multipole terms defining the magnetic multipole field of the main field coil. This synthesis is accomplished with a plurality of shimming coils which ideally exhibit pure multipole components.
In a common geometry, the field in the interior of the solenoid on or near the axis is the volume of interest for which the field distribution is to be controlled. The field may be resolved into radial and axial components for a field point at coordinates .rho.(.alpha..sub.1 =radius) and .theta. within a sphere centered on axis at the symmetry midplane and wholly within the magnet interior as ##EQU1## Where the quantities P.sub.n (cos .theta.) are Legendre polynominals of order n and the coefficients E.sub.n are Taylor series coefficients, as for example ##EQU2## For the special case of the axial field along the Z axis and the central plane respectively the above expressions reduce to ##EQU3##
The details of solenoid design are outside the scope of the present work. Relevant discussion may be found in Montgomery, Solenoid Magnet Design, Wiley Interscience, 1969.
Further departures from the desired spatial dependence follow from the construction of particular components. For example, the main field is obtained from a solenoid and additional axial and transverse components are usually present due to the construction of the solenoid. An example of the reduction of certain such gradients through construction details is given in U.S. Pat. No. 4,213,092.
In the case of an NMR spectrometer, the field in the interior of the solenoid is unidirectional of constant magnitude, but for small axial and radial gradients which it is desired to remove. The prior art construction of such solenoids and gradient removal is documented in U.S. Pat. Nos. 3,287,630; 3,419,904; 3,564,398; 3,577,067 and 4,180,769.
It is well known to workers in the field of NMR that the requisite shimming of the NMR magnetic field is often a tedious process due to the interaction of particular pairs of shim coils through the mutual inductance of such pairs of coils. An iterative procedure is generally required to arrive at the desired tolerance for the field dependence. In the case of persistent mode superconducting magnet systems, including superconducting shims, the operation is the more tedious for the requisite steps in altering the various persistent currents.
Superconducting magnet systems offer a further constraint due to the desired thermal isolation between the magnet in the interior of a cryostat and its external controlling apparatus.
In the prior art, it was recognized that selective excitation of the several independent components of a persistent mode magnet would offer an unusual improvement in reduction of thermal loss by minimizing the number of conductors required for communication between the interior and exterior of the magnet cryostat. Several embodiments directed to this end are discussed and claimed in U.S. Pat. No. 4,173,775, commonly assigned with the present work.
Earlier prior art provided leads sufficient to independently excite the several shim components independently. While this permits an iterative process of great flexibility, the number of conductors leading from the interior of the cryostat to ambient temperature undesirably reduces the thermal isolation between interior and exterior of the cryostat, thereby increasing the rate at which the cryogen is consumed by boiling.
It has also been found that the selective persistence switching function set out in U.S. Pat. No. 4,164,777 may be accomplished with a diode-based device where the heat evolved by the diode (forward bias) is sufficient to affect the transition of a portion of superconductor to its normal state. Although this prior art was inherently capable of exciting more than one persistence switch concurrently, no current limiting was provided in the individual switches. As a consequence, any selected switch could heat more than necessary for the switching function resulting in unnecessary dissipation of the cryogen.